1. Field of Invention
The invention relates to a laser beam scanner that scans a laser beam across a photosensitive member to form an image thereon.
2. Description of Related Art
Such a laser beam scanner 100, as shown in FIG. 7, has conventionally been known. FIG. 7 illustrates the basic structure of the conventional laser beam scanner 100.
The laser beam scanner 100 includes a laser light source 101 into which a semiconductor laser element and a collimator lens are integrated, a cylindrical lens 102 that converges the laser beam emitted from the laser light source 101, a polygon mirror 103 that deflects the laser beam having passed through the cylindrical lens 102, a first fθ lens 104 and a second fθ lens 105 that converge the laser beam deflected by the polygon mirror 103 on a photosensitive drum 110, a beam detector 106, as a light-sensing element, that detects the laser beam having passed through the second fθ lens 105, and a focusing lens 107 that converges the laser beam having passed through the second fθ lens 105 on the beam detector.
The first fθ lens 104 has converging power mainly in the main scanning direction, while the second fθ lens 105 has converging power mainly in the sub-scanning direction. At this time, it is to be understood that the main scanning direction is the direction in which the laser beam is scanned by the rotation of the polygon mirror 103, while the sub-scanning direction is the direction perpendicular to the main scanning direction. The beam detector 106, disposed at a laser beam converging point outside the effective scanning range on the photosensitive drum 110, detects the laser beam emitted from the laser light source 101 to provide the scan start timing.
In another known laser beam scanner, a mirror is provided to reflect the laser beam having passed through the second fθ lens 105 and the reflected laser beam is converged by the focusing lens 107 on the beam detector 106.
Also known is a multiple laser beam scanner that uses a plurality of laser beams in order to increase the exposure speed. In such a multiple laser beam scanner, a beam detector must receive all of the multiple laser beams, which are spaced from each other in the sub-scanning direction, in order to provide the scan start timing of each laser beam. In this case, the beam detector must have a light-receiving surface wide enough in the sub-scanning direction to receive all of the multiple laser beams.
However, as the light-receiving surface is increased, capacitance of the sensing element increases and its response deteriorates.
Sensitivity of the light-receiving surface varies from position to position depending on manufacturing conditions. When the positions of laser beams incident upon the light-receiving surface are fairly separated from each other, a lag of the scan start timing between the laser beams is caused due to the above-mentioned variations in sensitivity.
Another problem is that the large light-receiving surface makes the beam detector and its mounting space large. As a result, it is difficult to downsize products incorporating such a multiple laser beam scanner.